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(prō'təzō`ən), informal term for the unicellular heterotrophs of the kingdom ProtistaProtista
or Protoctista
, in the five-kingdom system of classification, a kingdom comprising a variety of unicellular and some simple multinuclear and multicellular eukaryotic organisms.
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. Protozoans comprise a large, diverse assortment of microscopic or near-microscopic organisms that live as single cells or in simple colonies and that show no differentiation into tissues. Formerly classified in the animal kingdom, they are now generally divided into five protist phyla: MastigophoraMastigophora
, phylum of unicellular heterotrophic protozoans of the kingdom Protista. Most of the approximately 1,500 species of Mastigophora are propelled by one or more flagella, and members of the group are sometimes referred to as flagellates.
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 (the flagellates), SarcodinaSarcodina,
the largest phylum (11,500 living species and 33,000 fossil species) of protozoans). It comprises the amebas and related organisms; which are all solitary cells that move and capture food by means of pseudopods, flowing temporary extensions of the cell.
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 (the amebas), CiliophoraCiliophora
, phylum in the kingdom Protista consisting of the ciliates, or ciliophores, complex freshwater or saltwater protozoans that swim by the coordinated beating of their cilia—short, hairlike structures that cover the cell surface.
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 (the ciliates), OpalinidaOpalinida
, phylum of unicellular heterotrophic organisms of the kingdom Protista. The opalinids are all intestinal parasites of small vertebrates, such as frogs, toads, and fish.
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, and SporozoaSporozoa
, phylum of unicellular heterotrophic organisms of the kingdom Protista. Unlike most other protozoans, sporozoans have no cilia or flagella. All species are parasitic and have elaborate life cycles, often requiring more than one host.
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. Most are motile, and most ingest food, as do animals, rather than produce it themselves, as do plants. The 26,000 living species are cosmopolitan in distribution; they are found in freshwater and at all depths in the ocean; some live in soil. Some are parasites in the bodies of humans or other animals, sometimes causing diseases.

Cellular Structure and Function

The various forms have in common a unicellular structure consisting of a mass of cytoplasm with one or more nuclei (see cellcell,
in biology, the unit of structure and function of which all plants and animals are composed. The cell is the smallest unit in the living organism that is capable of integrating the essential life processes. There are many unicellular organisms, e.g.
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). Like all cells, they are bounded by a thin cell membrane; in addition, most have a tough outer membrane called a pellicle, which maintains their form. Despite their small size and lack of organization into multicellular systems, protozoans carry on all the metabolic functions of animals. Organelles, or intracellular structures, carry out a variety of functions, such as digestion, excretion, respiration, and coordination of movement; some protozoans are much more complex in their internal structure than are the cells of multicellular animals.


Some protozoans have complex digestive systems and feed on large food particles, such as other microorganisms. The food is digested by means of enzymes and the wastes transported to the cell surface or stored in vacuoles (bubblelike spaces in the cytoplasm). Others have no digestive system and absorb dissolved organic matter through the cell membrane.


Respiration is accomplished by the diffusion of dissolved gases through the cell membrane. Oxygen diffuses into the cell, where it oxidizes food molecules, producing energy and the organic molecules used for the building and maintenance of the cell. Carbon dioxide and water, the waste products of this oxidation, diffuse out of the cell.


Reproduction is usually asexual, occurring mostly by cell division, or binary fission; some forms reproduce asexually by budding or by the formation of spores (reproductive cells that give rise to a new organism without fertilization). In certain groups sexual reproduction sometimes also occurs. In these instances, cell division is preceded by the fusion of two individuals or, in ciliates, by conjugation and exchange of nuclear material.


See H. Curtis, The Marvelous Animals (1968); T. Jahn, How to Know the Protozoa (2d ed. 1978); J. J. Lee, S. H. Hunter, and E. C. Bovee, An Illustrated Guide to the Protozoa (1985); M. Sleigh, Protozoa and Other Protists (1989).


Also called protozoon any of various minute unicellular organisms formerly regarded as invertebrates of the phylum Protozoa but now usually classified in certain phyla of protoctists. Protozoans include flagellates, ciliates, sporozoans, amoebas, and foraminifers
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Cryptococcus neoformans, the etiology of cryptosporidioisis is a minute, intracellular apicomplexan protozoal parasite (Figure 3).
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Also, the few antibacterial compounds effective against viral, fungal, and protozoal compounds are mentioned.
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ABSTRACT The eastern oyster is an economically and ecologically important species whose vitality is threatened by the protozoal parasite Perkinsus marinus.
Rats are also host to four forms of cellular protozoal parasites including those causing a condition called toxoplasmosis.